Digestive and Respiratory Systems: Types and Functions

Types of Digestion

Digestion is the process by which complex molecules are broken down into simpler ones. There are two methods of digestion:

• Intracellular Digestion: This takes place inside cells. Each cell functions as a complete digestive system:
  • Food capture (pseudopodia, cilia, or flagella)
  • Food inclusion (endocytosis)
  • Food digestion (lysosomes)
  • Expulsion of undigested material (exocytosis)
  Typical of sponges.
• Extracellular Digestion: This is performed inside the digestive systems consisting of one tract and specialized glands. The nutrients pass into the bloodstream and are distributed to reach all cells. Typical of most animals.

Some animals combine both types of assimilation:

• The food comes in the form of large fragments.
• The inner wall of the digestive system contains cells that produce enzymes, causing them to make a first digestion (extracellular digestion).
• Small molecules pass into the cells, completing the final digestion (intracellular digestion).

Typical of coelenterates.

Types of Lungs

There are two types of lungs:

• Lung Diffusion: These have no ventilation mechanisms. Found in terrestrial invertebrates.
• Lung Ventilation: Found in vertebrates. Ventilation is performed in two acts: inspiration and expiration.

Breathing in Amphibians

The lungs of amphibians are two sacs with thin, elastic walls.

Respiration in Reptiles

Reptile lungs are more developed than in amphibians and have a rib cage.

Breathing in Birds

The respiratory system of birds is very special because the bronchi are divided into parabronchi. Birds have one-way ventilation (air goes in one side and out the other).

Breathing in Mammals

In mammals, the lungs are housed in the rib cage and are limited by the diaphragm.

Formation of Urine

The simplest organisms remove waste products directly into the aqueous medium. As complexity increases, excretion occurs, and the product is urine.

The formation of urine involves three processes:

• Filtration: Along with waste products, useful substances for the body are filtered. The result of the filtration is primary urine.
• Resorption: Recovery of substances of interest to retain.
• Discharge: Some substances are added to the urine at the end of the process.